[41.09] Aperture Corrections for WFPC2 Stellar Photometry

A. Suchkov (STScI)

We present photometric corrections to compensate for the systematic
change in focus position of the Hubble Space Telescope (HST) during its
operations. The metering truss of HST has shrunk regularly over
the years, presumably because of water desorption, at an average rate of
about 0.75 \mu m/month over the active life of WFPC2. The shrinkage
has been compensated by regular movements of the HST secondary mirror,
ensuring that the focus has always been within 3 \mu m of the
nominal value. The shrinkage of the metering truss has been much slower
over the last year or so, so that future focus adjustments will be
smaller and less frequent.

In between secondary mirror adjustments, the accumulating focus drift
results in changing aperture corrections, which impact small aperture
photometry. We have derived aperture corrections phased with the focus
drift using the WFPC2 data accumulated over three years of observations of
a standard star. The correction is well described by a quadratic fit
to the nominal focus position, with an amplitude that depends on
the filter, the chip and the aperture used. The fitted correction
can be used to better assess photometric accuracy and to adjust the
photometry, if needed.

Using composites of tens of standard star images, we have also derived and
tabulated mean aperture corrections to large aperture radii (``infinity'')
for some of the most often used WFPC2 filters. The corrections may be
helpful in estimating the amount of light beyond the photometry aperture
in cases when large apertures cannot be used.